Semiconductor diode lasers are formed of multiple layers of semiconductor materials. The typical semiconductor diode laser includes an n-type layer, a p-type layer and an undoped active layer between them such that when the diode is forward-biased, electrons and holes recombine in the active region layer with the resulting emission of light. The active layers (quantum well(s) resides in the waveguide layer, which has a higher index of refraction than the cladding layers that confine the emitted light to the waveguide and the active layers . Semiconductor lasers may be constructed to be either edge emitting or surface emitting. In an edge emitting Fabry-Perot (FP) type semiconductor laser, crystal facet mirrors are located at opposite edges of the multi-layer structure to provide reflection of the emitted light back and forth in a longitudinal direction, generally in the plane of the layers, to provide generation of laser light (lasing) action and emission of laser light from one of the facets. Another type of device, which may be designed to be either edge emitting or surface emitting, utilizes distributed feedback (DFB) structures rather than conventional facets or mirrors, providing feedback for lasing as a result of backward Bragg scattering from periodic variations of the refractive index or the gain or both of the semiconductor laser structure ensuing in narrowed emission bandwidth for the output spectrum and lower sensitivity to wavelength drift due to temperature variation.
High power diode lasers have been extensively used for pumping high power solid-state lasers such as the thin disk, slab, rod, micro-chip and fiber lasers that are useful for industrial, printing, medical applications and scientific instrumentation. There are also emerging alkali-vapor gas lasers that are pumped by semiconductor lasers. Multimode 975 nm diode lasers are of particular interest for pumping the upper transition states of rare-earth doped (such as Yb, Er and Yb/Er co-doped) solid state lasers, fiber lasers and amplifiers. At this pump wavelength, the quantum defect is minimal and the absorption cross-section is much higher (2.5 dB/m) relative to the 920 nm transition states (0.7 dB/m). Hence, shorter gain fibers may be used to mitigate deleterious nonlinear effects such as the Stimulated Raman Scattering (SRS) and the Stimulated Brillouin Scattering (SBS) that can occur in high average or peak power application. However, the absorption bandwidth at 975 nm is quite narrow (<9 nm FWHM). Similarly, the absorption peak is narrow for other solid-state host materials with Yb, Er or Yb/Er co-doping. Nd-doped solid-state gain media such as Nd:YAG also has narrow absorption cross-section near 808 nm and 885 nm absorption bands. As a result, either expensive thermal stabilization measures or very sensitive external wavelength-locking methods such as use of diffraction gratings, Fiber Bragg Gratings (FBG) or Volume Bragg Gratings (VBG) in an external cavity configuration have to be employed making diode lasers less attractive as pump sources for these applications to pump at the narrow gain band region. A monolithic integration of Bragg grating inside the semiconductor laser cavity is a simpler and a more cost-effective means of achieving both the wavelength stabilization as well as emission linewidth narrowing making multimode DFB laser an attractive pump source for the aforementioned precision-pumping applications.
Semiconductor lasers having continuous wave (CW) power in the several watt-range and narrow bandwidth, e.g., less than 3 Å. full width half maximum (FWHM), would be desirable for a variety of applications. Conventional, FP broad stripe (≧25 μm) semiconductor lasers used for obtaining high powers typically have a spectral width of about 20 Å FWHM or more at high drive levels and broaden further under quasi-CW operation. Since the lasing wavelength in FP diode laser is determined by the peak wavelength of the gain spectrum, the center of the lasing wavelength shifts as a function of temperature. This temperature tuning rate is approximately 0.32 nm per Centigrade. Significant improvements in spectral width and temperature tuning rate can be obtained using distributed feedback (DFB) gratings as reported by M. Kanskar, et al, Electron. Lett. Vol. 41, p. 33, 2005, or distributed Bragg reflectors (DBR) rather than FP mirror facets for optical feedback. A CW power of about 278 mW with about 1 Å of wavelength variation, resulting from mode hopping, has been reported for narrow-stripe DBR lasers. J. S. Major, et al., Electron. Lett. Vol. 29, No. 24, p. 2121, 1993. Using DFB phase-locked laser arrays, narrow bandwidth operation has been obtained from large apertures at relatively long wavelengths (λ=1.3 μm to 1.5 μm). Pulsed operation at a power level of 120 mW has been reported from a 45 μm aperture device (λ=1.3 μm), Y. Twu, et al., Electron. Lett. Vol. 24, No. 12, p. 1144, 1988, and 85 mW CW from a 72 μm aperture device (λ=1.55 μm), K. Y. Liou, et al., Tech. Dig. 13th IEEE Int. Semicond. Laser Conf., Paper D7, 1992. For applications where (lateral) spatial coherence is not necessary, a broad-stripe laser with a DFB grating is apparently well suited for achieving high CW powers with narrow spectral linewidth and more robust temperature tuning characteristics.
For diode lasers operating in the near infrared spectral region, it is simple and cost-effective to fabricate a second-order grating since the grating pitch is typically submicron in length. However, when a second-order distributed feedback laser is fabricated there is an additional optical power loss incurred compared to a FP or the first-order DFB laser. This problem arises because the second-order grating has a first-order diffraction that scatters light out in directions that are normal to the propagation direction of the fundamental mode. As a result, the differential quantum efficiency (DQE) is usually lower than that for the FP or the first-order DFB laser making the power-conversion efficiency of a second-order DFB laser poorer.
It is known that magnitude of the first-order diffraction loss in DFB lasers can also be minimized by reducing the index contrast of the grating and/or by placing the grating far away from the peak of the transverse optical intensity, as discussed in U.S. Pat. No. 6,455,341 by Macomber. As disclosed by Macomber, the first-order diffraction loss is minimized by introducing low index contrast which leads to lower scattering strength for the grating thereby reducing scattering loss. Additionally, by locating the grating where the transverse optical intensity is lower, the fraction of the diffracted light is reduced. This technique pertains to minimization of the transverse optical field only and does not address the issue of continual diffraction loss which occurs during propagation along the longitudinal direction as the laser light oscillates back and forth numerous times inside the DFB laser cavity.
Currently, the most straightforward method to overcome diffraction loss from the laser cavity is to introduce a first-order grating in the laser cavity. As a result, there are no possible diffraction orders that could lead to radiation loss from the cavity modes. However, making a first-order grating can be impractical and expensive, especially for short wavelength radiation. In order to overcome this practical problem, a second-order grating that is distributed over the entire gain volume (e.g. laser cavity) is used to both stabilize and narrow the emission bandwidth of a laser. While this method stabilizes the wavelength and narrows the emission bandwidth, the second-order grating distributed across the entire laser cavity leads to continuous first-order diffraction loss of radiation out of the cavity as the laser modes oscillate back and forth inside the resonator. Monolithic distributed Bragg reflector (DBR) lasers have also been used in the past to stabilize and lock the wavelength, such as, for example, TOPTICA PHOTONICS (Westfield, Mass.). This technique has an additional disadvantage. The DBR section is not electrically pumped; hence, the gain section underneath the DBR acts as a saturable absorber, reducing the overall efficiency of the laser. Additionally, the use of a saturable absorber can also lead to a deleterious effect known as self-pulsation.
FIG. 1 illustrates some concepts of a conventional distributed feedback laser 40. As shown and used herein, the term laser “cavity” 12 refers to the space between the high reflection coating (HR) 14 and the anti-reflection coating (AR) 16 of the laser. As used herein, the term “laser facet” refers to the facet holding the HR and AR coating. Thus, the back, or rear facet 18 is defined by the HR coating and the front facet 20 is defined by the AR coating. A grating 42 is a dielectric layer with a periodic perturbation of the refractive index so that sufficient reflectivity may be reached at a wavelength due to Bragg scattering which provides optical feedback for the lasing to be established. Gratings are constructed to reflect only a narrow band of wavelengths and thus, produce a narrow linewidth of laser output.
It would be desirable however, to further reduce first-order scattering loss in a distributed feedback laser that uses a second-order grating so that a maximum possible efficiency could be achieved.